linux-kernel - Re: [PATCH v6 2/2] printk: Make printing of spin

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Message-ID: <20160314024054.GC8074@swordfish>
Date:	Mon, 14 Mar 2016 11:40:55 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Byungchul Park <byungchul.park@....com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	akpm@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	peter@...leysoftware.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org
Subject: Re: [PATCH v6 2/2] printk: Make printing of spin_dump() deferred to
 avoid a deadlock

Hi,

On (03/14/16 11:30), Byungchul Park wrote:
[..]
> > so can it be
> > 
> > vprintk_emit()
> >  __spin_dump_deferred()
> >   vprintk_deferred()
> >    vprintk_emit()
> >     __spin_dump_deferred()
>       ^^^
>       can be caused by raw_spin_lock(logbug_lock)
> 
> >      vprintk_deferred()
> 
> Yes, it can happen by raw_spin_lock(logbuf_lock) to print warning or error
> message. Are you worrying about an infinite recursion?

yes.

> 1. In the case printing warning, eventually it can fill the buffer without
> actual printing using console.

so the worry is that the CPU that spins on __spin_dump_deferred() has IRQs
disabled and `printk_pending' bit set; but IRQ may never be enabled on this
CPU.

> 2. In the case printing error, the infinite recursion can be prevented by
> debug_locks_off().
> 
> Therefore, no problem.

	-ss